Transistor

ABSTRACT

It is an object to provide a thin film transistor with high speed operation, in which a large amount of current can flow when the thin film transistor is on and off-state current is extremely reduced when the thin film transistor is off. The thin film transistor is a vertical thin film transistor in which a channel formation region is formed using an oxide semiconductor film in which hydrogen is contained in an oxide semiconductor at a concentration of lower than or equal to 5×10 19 /cm 3 , preferably lower than or equal to 5×10 18 /cm 3 , more preferably lower than or equal to 5×10 17 /cm 3 , hydrogen or an OH group contained in the oxide semiconductor is/are removed, and carrier concentration is lower than or equal to 5×10 14 /cm 3 , preferably lower than or equal to 5×10 12 /cm 3 .

TECHNICAL FIELD

The present invention relates to a field-effect transistor, for examplea thin film transistor formed using an oxide semiconductor.

BACKGROUND ART

A lot of attention has been paid to the technology of forming afield-effect transistor, for example a thin film transistor (TFT), byusing a semiconductor thin film that is formed over a substrate havingan insulating surface. The thin film transistor is used for a displaydevice typified by a liquid crystal television. Although silicon-basedsemiconductor material is known as a material for a semiconductor thinfilm which can be applied to the thin film transistor, an oxidesemiconductor has attracted attention as another material.

As the material for the oxide semiconductor, zinc oxide and a materialcontaining zinc oxide are known. A thin film transistor formed using anamorphous oxide (oxide semiconductor) whose electronic carrierconcentration is lower than 10¹⁸/cm³ is disclosed (Patent Documents 1 to3).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165527-   [Patent Document 2] Japanese Published Patent Application No.    2006-165528-   [Patent Document 3] Japanese Published Patent Application No.    2006-165529

DISCLOSURE OF INVENTION

Deviation from the stoichiometric composition of an oxide semiconductorarises in a thin film formation process. For example, electricalconductivity of an oxide semiconductor is changed because of excess anddeficiency of oxygen. Further, hydrogen which is mixed in an oxidesemiconductor during thin film formation is used to form oxygen(O)-hydrogen (H) bonds to serve as an electron donor, which is a factorfor changing the electric conductivity. Since an OH group is polarized,an OH bond is a factor for changing characteristics of an active devicesuch as a thin film transistor formed using an oxide semiconductor.

Even when the electronic carrier concentration is lower than 10¹⁸/cm³,an oxide semiconductor is a substantially n-type oxide semiconductor,and ON/OFF ratio of the thin film transistors disclosed in the abovePatent Documents is just 10³. The reason the ON/OFF ratio of such a thinfilm transistor is low is that off-state current is high.

In the display device, there is a problem in that unwanted chargebuild-up is caused in elements, electrodes, or wirings duringmanufacture or operation. In the case of a thin film transistor, forexample, such charge build-up will generate a parasitic channel whichallows leakage current to flow. Further, in the case where a bottom gatetransistor is used, charge may build up on a surface of or in a backchannel portion in a semiconductor layer (i.e., a region of asemiconductor layer which is sandwiched between a source electrode and adrain electrode which are formed over the semiconductor layer) andgenerate a parasitic channel, so that leakage current is likely to occurand threshold voltage varies.

In order to increase field-effect mobility of a thin film transistor,the channel length along which carriers transfer is preferably small.However, when the channel length is small, off-state current of the thinfilm transistor is increased.

Thus, it is an object of one embodiment of the present invention toprovide a thin film transistor with high speed operation, in which alarge mount of current can flow when the thin film transistor is on andoff-state current is extremely reduced when the thin film transistor isoff.

One embodiment of the present invention is a vertical thin filmtransistor in which a channel formation region is formed using an oxidesemiconductor which has a larger energy gap than a silicon semiconductorand which is an intrinsic or substantially intrinsic semiconductor byremoval of impurities which can be an electron donor (donor) in theoxide semiconductor.

That is, one embodiment of the present invention is a vertical thin filmtransistor in which a channel formation region is formed using an oxidesemiconductor film in which hydrogen is contained in an oxidesemiconductor at a concentration of lower than or equal to 5×10¹⁹/cm³,preferably lower than or equal to 5×10¹⁸/cm³, more preferably lower thanor equal to 5×10¹⁷/cm³, and hydrogen or an OH group contained in theoxide semiconductor is/are removed, and carrier concentration is lowerthan or equal to 5×10¹⁴/cm³, preferably lower than or equal to5×10¹²/cm³.

The energy gap of the oxide semiconductor is greater than or equal to 2eV, preferably greater than or equal to 2.5 eV, more preferably greaterthan or equal to 3 eV. The impurities such as hydrogen which forms adonor are reduced as much as possible. The carrier concentration islower than or equal to 1×10¹⁴/cm³, preferably lower than or equal to1×10¹²/cm³.

Note that in one embodiment of the present invention, a gate electrodeof the thin film transistor has a ring shape and surrounds a sourceelectrode, an oxide semiconductor film, and a drain electrode with agate insulating film interposed therebetween. That is, the gateelectrode faces side surfaces of the source electrode, the oxidesemiconductor film, and the drain electrode with the gate insulatingfilm interposed therebetween. Therefore, the channel width is large.

According to one embodiment of the present invention, an oxidesemiconductor whose hydrogen concentration is reduced and purity isincreased is used, so that field-effect mobility and an on-state currentof the field-effect transistor, for example the thin film transistor canbe improved, and very low off-state current can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are a top view and a cross-sectional view illustrating athin film transistor.

FIG. 2 is a longitudinal cross-sectional view of an inverted staggeredthin film transistor formed using an oxide semiconductor.

FIGS. 3A and 3B are energy band diagrams (schematic diagrams) of an A-A′cross section illustrated in FIG. 2.

FIG. 4A is a diagram illustrating a state where a positive potential(+V_(G)) is applied to a gate (GE1), and FIG. 4B is a diagramillustrating a state where a negative potential (−V_(G)) is applied tothe gate (GE1).

FIG. 5 is a diagram illustrating a relation between a vacuum level andwork function (φ_(M)) of metal and a relation between a vacuum level andelectron affinity (χ) of an oxide semiconductor.

FIG. 6A is a top view illustrating a thin film transistor and FIG. 6B isa cross-sectional view illustrating the thin film transistor.

FIGS. 7A to 7E are cross-sectional views illustrating a method formanufacturing a thin film transistor.

FIGS. 8A and 8B are cross-sectional views illustrating a method formanufacturing a thin film transistor.

FIG. 9 is a top view illustrating a pixel of a display device.

FIG. 10 is a cross-sectional view illustrating a pixel of a displaydevice.

FIGS. 11A-1 and 11A-2 are plan views of semiconductor devices, and FIG.11B is a cross-sectional view of a semiconductor device.

FIG. 12 is a cross-sectional view of a semiconductor device.

FIG. 13A is a plan view of a semiconductor device and FIG. 13B is across-sectional view of the semiconductor device.

FIGS. 14A to 14C are views each illustrating an electronic device.

FIGS. 15A to 15C are views each illustrating an electronic device.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below withreference to the accompanying drawings. Note that the present inventionis not limited to the following description, and it will be easilyunderstood by those skilled in the art that various changes andmodifications can be made without departing from the spirit and scope ofthe present invention. Therefore, the present invention should not beconstrued as being limited to the description in the followingembodiments. Note that in structures of the present invention describedhereinafter, like portions or portions having similar functions aredenoted by the same reference numerals in different drawings, anddescription thereof is not repeated.

Note that in each drawing described in this specification, a size ofeach component or a thickness of each layer or an area is exaggerated insome cases for clarification. Therefore, embodiments of the presentinvention are not limited to such scales.

Note that the numeral terms such as “first”, “second”, and “third” inthis specification are used in order to avoid confusion betweencomponents and do not set a limitation on number. Therefore, forexample, description can be made even when “first” is replaced with“second” or “third”, as appropriate.

Note that “voltage” indicates a difference between potential of twopoints, and “potential” indicates electrostatic energy (electricalpotential energy) of a unit charge at a given point in an electrostaticfield. Note that in general, an electric potential of difference betweena potential of one point and a reference potential (e.g. a groundpotential) is merely called a potential or voltage, and a potential andvoltage are used as synonymous words in many cases. Thus, in thisspecification, a potential may be rephrased as voltage and voltage maybe rephrased as a potential unless otherwise specified.

Embodiment 1

In this embodiment, a structure of a field-effect transistor, forexample a thin film transistor will be described with reference to FIGS.1A and 1B.

FIG. 1A is a top view of a thin film transistor 145, and FIG. 1Bcorresponds to a cross-sectional view of dashed line A-B of FIG. 1A.

As illustrated in FIG. 1B, a first electrode 105, an oxide semiconductorfilm 107, and a second electrode 109 are stacked over an insulating film103 formed over a substrate 101. A gate insulating film 111 is providedso as to cover the first electrode 105, the oxide semiconductor film107, and the second electrode 109. A third electrode 113 is providedover the gate insulating film 111. An insulating film 117 functioning asan interlayer insulating film is provided over the gate insulating film111 and the third electrode 113. Openings are formed in the insulatingfilm 117, and a wiring 131 (see FIG. 1A) connected via the opening tothe first electrode 105, a wiring 129 connected via the opening to thesecond electrode 109, and a wiring 125 connected via the opening to thethird electrode 113 are formed.

The first electrode 105 functions as one of a source electrode and adrain electrode of the thin film transistor 145. The second electrode109 functions as the other of the source electrode and the drainelectrode of the thin film transistor 145. The third electrode 113functions as a gate electrode of the thin film transistor 145.

In this embodiment, the third electrode 113 functioning as the gateelectrode has a ring shape. When the third electrode 113 functioning asthe gate electrode has a ring shape, the channel width of the thin filmtransistor can be increased. Accordingly, on-state current of the thinfilm transistor can be increased.

Note that a thin film transistor is an element that includes at leastthree terminals, including a gate, a drain, and a source. The thin filmtransistor includes a channel formation region between a drain regionand a source region, and current can flow through the drain region, thechannel formation region, and the source region. Here, since the sourceand the drain of the thin film transistor may change depending on astructure, operating conditions, and the like of the thin filmtransistor, it is difficult to define which is a source or a drain.Therefore, a region functioning as a source and a drain is not calledthe source or the drain in some cases. In such a case, for example, oneof the source and the drain may be referred to as a first terminal andthe other thereof may be referred to as a second terminal.Alternatively, one of the source and the drain may be referred to as afirst electrode and the other thereof may be referred to as a secondelectrode. Further alternatively, one of the source and the drain may bereferred to as a first region and the other thereof may be referred toas a second region.

It is necessary that the substrate 101 at least have enough heatresistance to heat treatment to be performed later. As the substrate101, a glass substrate of barium borosilicate glass, aluminoborosilicateglass, or the like can be used.

As the glass substrate, in the case where the temperature of the heattreatment to be performed later is high, the one whose strain point ishigher than or equal to 730° C. is preferably used. As a glasssubstrate, a glass material such as aluminosilicate glass,aluminoborosilicate glass, or barium borosilicate glass is used, forexample. Note that in general, by containing a larger amount of bariumoxide (BaO) than boric acid, a glass substrate which is heat-resistantand more practical can be obtained. Therefore, a glass substratecontaining BaO and B₂O₃ so that the amount of BaO is larger than that ofB₂O₃ is preferably used.

Note that a substrate formed of an insulator, such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate, may be used,instead of the glass substrate. Alternatively, a crystallized glasssubstrate or the like may be used.

The insulating film 103 is formed using an oxide insulating film such asa silicon oxide film or a silicon oxynitride film; or a nitrideinsulating film such as a silicon nitride film, a silicon nitride oxidefilm, an aluminum nitride film, or an aluminum nitride oxide film. Inaddition, the insulating film 103 may have a stacked structure, forexample, a stacked structure in which one or more of the nitrideinsulating films and one or more of the oxide insulating film arestacked in that order over the substrate 101.

The first electrode 105 and the second electrode 109 are formed using anelement selected from aluminum, chromium, copper, tantalum, titanium,molybdenum, tungsten, or yttrium; an alloy containing any of theseelements as a component; an alloy containing these elements incombination; or the like. Alternatively, one or more materials selectedfrom manganese, magnesium, zirconium, and beryllium can be used. Inaddition, the first electrode 105 can have a single-layer structure or astacked structure having two or more layers. For example, a single-layerstructure of an aluminum film containing silicon; a two-layer structureof an aluminum film and a titanium film stacked thereover; a two-layerstructure of a tungsten film and a titanium film stacked thereover; athree-layer structure in which a titanium film, an aluminum film, and atitanium film are stacked in that order; and the like can be given.Alternatively, a film, an alloy film, or a nitride film which containsaluminum and one or more elements selected from titanium, tantalum,tungsten, molybdenum, chromium, neodymium, and scandium may be used.

Note that as the oxide semiconductor film 107, a thin film expressed byInMO₃(ZnO)_(m) (m>0, where m is not necessarily an integer) can be used.Here, M represents one or more metal elements selected from Ga, Fe, Ni,Mn, and Co. For example, M may be Ga, Ga and Ni, Ga and Fe, or the like.The oxide semiconductor film may contain a transition metal element oroxide of the transition metal element as an impurity element in additionto the metal element contained as M. Among oxide semiconductor layerswhose composition formulae are expressed by InMO₃(ZnO)_(m) (m>0 and m isnot an integer), an oxide semiconductor which contains Ga as M isreferred to as an In—Ga—Zn—O-based oxide semiconductor, and a thin filmof the In—Ga—Zn—O-based oxide semiconductor is referred to as anIn—Ga—Zn—O-based film.

As the oxide semiconductor film 107, any of the following oxidesemiconductor films can be applied as well as the above In—Ga—Zn—O-basedfilm: an In—Sn—Zn—O-based oxide semiconductor film; an In—Al—Zn—O-basedoxide semiconductor film; a Sn—Ga—Zn—O-based oxide semiconductor film;an Al—Ga—Zn—O-based oxide semiconductor film; an Sn—Al—Zn—O-based oxidesemiconductor film; an In—Zn—O-based oxide semiconductor film; aSn—Zn—O-based oxide semiconductor film; an Al—Zn—O-based oxidesemiconductor film; an In—O-based oxide semiconductor film; a Sn—O-basedoxide semiconductor film; and a Zn—O-based oxide semiconductor film.Further, the oxide semiconductor film may further contain Si.

Hydrogen contained in the oxide semiconductor is removed, so that theconcentration of hydrogen in the oxide semiconductor film 107 used inthis embodiment is lower than or equal to 5×10¹⁹/cm³, preferably lowerthan or equal to 5×10¹⁸/cm³, more preferably lower than or equal to5×10¹⁷/cm³. That is, the oxide semiconductor film is highly purified sothat impurities that are not main components of the oxide semiconductorfilm may be contained as little as possible. The carrier concentrationof the oxide semiconductor film 107 is lower than or equal to5×10¹⁴/cm³, preferably lower than or equal to 1×10¹⁴/cm³, morepreferably lower than or equal to 5×10¹²/cm³, further preferably lowerthan or equal to 1×10¹²/cm³. That is, the carrier concentration of theoxide semiconductor film is as close to zero as possible. Furthermore,the energy gap is greater than or equal to 2 eV, preferably greater thanor equal to 2.5 eV, more preferably greater than or equal to 3 eV. Notethat the hydrogen concentration in the oxide semiconductor film can bedetected by secondary ion mass spectroscopy (SIMS). The carrierconcentration can be measured by the Hall effect measurement.

In this embodiment, since the shape of the top surface of the oxidesemiconductor film of the transistor is rectangular, the channel width Wis the sum of 2W₁ and 2W₂. Note that in the case where the shape of thetop surface of the oxide semiconductor film of the transistor iscircular, the channel width W is 2πr where r is a radius of the oxidesemiconductor film.

The thickness of the oxide semiconductor film 107 may be greater than orequal to 30 nm and less than or equal to 3000 nm. When the thickness ofthe oxide semiconductor film 107 is small, the channel length L of thethin film transistor can be decreased; thus a thin film transistorhaving high on-state current and high field-effect mobility can bemanufactured. On the other hand, when the thickness of the oxidesemiconductor film 107 is large, typically greater than or equal to 100nm and less than or equal to 3000 nm, a semiconductor device for highpower can be manufactured.

The gate insulating film 111 can be a single-layer or a stack formedusing a silicon oxide film, a silicon nitride film, a silicon oxynitridefilm, a silicon nitride oxide film, or an aluminum oxide film. A portionof the gate insulating film 111 which is in contact with the oxidesemiconductor film 107 preferably contains oxygen, and in particular,the gate insulating film 111 is preferably formed using a silicon oxidefilm. A silicon oxide film is used, so that oxygen can be supplied tothe oxide semiconductor film 107 and favorable characteristics can beobtained.

The gate insulating film 111 is formed using a high-k material such ashafnium silicate (HfSiO_(x) (x>0)), HfSiO_(x)N_(y) (x>0, y>0), hafniumaluminate (HfAlO_(x) (x>0)), hafnium oxide, or yttrium oxide, so thatgate leakage current can be reduced. Further, a stacked structure can beused in which a high-k material and one or more of a silicon oxide film,a silicon nitride film, a silicon oxynitride film, a silicon nitrideoxide film, and an aluminum oxide film are stacked. The thickness of thegate insulating film 111 may be greater than or equal to 50 nm and lessthan or equal to 500 nm. When the thickness of the gate insulating film111 is small, a thin film transistor having high field-effect mobilitycan be manufactured; thus a driver circuit can be manufactured on thesame substrate as the thin film transistor. In contrast, when thethickness of the gate insulating film 111 is large, gate leakage currentcan be reduced.

The third electrode 113 functioning as a gate electrode is formed usingan element selected from aluminum, chromium, copper, tantalum, titanium,molybdenum, or tungsten; an alloy containing any of these elements as acomponent; an alloy film containing these elements in combination; orthe like. Alternatively, one or more materials selected from manganese,magnesium, zirconium, or beryllium may be used. In addition, the thirdelectrode 113 can have a single-layer structure or a stacked structurehaving two or more layers. For example, a single-layer structure of analuminum film containing silicon; a two-layer structure of an aluminumfilm and a titanium film stacked thereover; a three-layer structure inwhich a titanium film, an aluminum film, and a titanium film are stackedin that order; and the like can be given. Alternatively, a film, analloy film, or a nitride film which contains aluminum and one or moreelements selected from titanium, tantalum, tungsten, molybdenum,chromium, neodymium, or scandium may be used.

Next, operation of the thin film transistor having the oxidesemiconductor film 107 will be described with reference to energy banddiagrams.

FIG. 2 is a longitudinal cross-sectional view of an inverted staggeredthin film transistor formed using the oxide semiconductor film describedin this embodiment. An oxide semiconductor film (OS) and a sourceelectrode (S) are stacked over a drain electrode (D). A gate insulatingfilm (GI) is provided over the drain electrode, the oxide semiconductorfilm, and the source electrode, and a gate electrode (GE1) is providedthereover.

FIGS. 3A and 3B are energy band diagrams (schematic diagrams) of across-section taken along line A-A′ in FIG. 2. FIG. 3A illustrates anenergy band diagram (schematic diagram) in the case where voltagebetween the source and the drain is set to (V_(D)=0V), and FIG. 3Billustrates an energy band diagram (schematic diagram) of across-section taken along line A-A′ in FIG. 2. Dashed lines show thecase where voltage is not applied to the gate (V_(G)=0) under thecondition such that positive voltage (V_(D)>0) is applied to the drain,and solid lines show the case where positive voltage (V_(G)>0) isapplied to the gate under the condition such that positive voltage(V_(D)>0) is applied to the drain. In the case where voltage is notapplied to the gate, a carrier (electron) is not injected from theelectrode to the oxide semiconductor side because of the high potentialbarrier, so that current does not flow, which means an off state. On theother hand, when positive voltage is applied to the gate, a potentialbarrier is lowered, so that current flows, which means an on state.

FIGS. 4A and 4B are energy band diagrams (schematic diagrams) of across-section taken along line B-B′ in FIG. 2. FIG. 4A illustrates astate where a positive potential (+V_(G)) is applied to the gate (GE1),that is, an on state (conducting state) where a carrier (electron) flowsbetween a source and a drain. FIG. 4B illustrates a state where anegative potential (−V_(G)) is applied to the gate (GE1), that is, anoff state (a non-conducting state or a state where a minority carrierdoes not flow).

FIG. 5 illustrates a relation between a vacuum level and work function(φ_(M)) of metal and a relation between a vacuum level and electronaffinity (χ) of an oxide semiconductor.

Free electron of metal is in a degenerated state, and the Fermi level islocated in a conduction band. On the other hand, a conventional oxidesemiconductor film is generally of n-type, and the Fermi level (E_(F))in that case is located close to the conduction band and is away fromthe intrinsic Fermi level (Ei) located in the middle of the band gap. Itis known that part of hydrogen contained in the oxide semiconductor filmserves as a donor and is a factor of making the oxide semiconductor filmn-type.

In contrast, in the oxide semiconductor film according to thisembodiment, hydrogen which is an n-type impurity is removed from theoxide semiconductor film, and the oxide semiconductor film is highlypurified so that impurities that are not main components of the oxidesemiconductor film may be contained as little as possible, whereby theoxide semiconductor film is an intrinsic (i-type) oxide semiconductorfilm or a substantially intrinsic oxide semiconductor film. That is, thehighly purified oxide semiconductor film is or is a substantially i-type(intrinsic) oxide semiconductor film not by addition of impurities to bean i-type oxide semiconductor film but by removal of impurities such ashydrogen, water, hydroxyl groups, or hydride as much as possible.Accordingly, the Fermi level (E_(F)) can be made to the same level asthe intrinsic Fermi level (Ei).

It is said that electron affinity (χ) is 4.3 eV when the band gap (Eg)of the oxide semiconductor film is 3.15 eV. The work function oftitanium (Ti) which forms the source electrode and the drain electrodeis approximately equal to the electron affinity (χ) of the oxidesemiconductor film. In the case where titanium is used for the sourceelectrode and the drain electrode, a Schottky barrier to electrons isnot formed at the interface between the metal and the oxidesemiconductor film.

In other words, an energy band diagram (schematic diagram) like FIG. 3Ais obtained in the case where the metal and the oxide semiconductor filmare in contact with each other when the work function of the metal(φ_(M)) is approximately equal to the electron affinity (χ) of the oxidesemiconductor film.

In FIG. 3B, a black circle () represents an electron. When a positivepotential is applied to the drain, the electron is injected into theoxide semiconductor film over a barrier (h) and flows toward the drain.In this case, the height of the barrier (h) changes depending on thegate voltage and drain voltage. When positive drain voltage is applied,the height of the barrier (h) is smaller than the height of the barrier(h) without application of voltage in FIG. 3A, namely, a half of theband gap (Eg).

In this case, as shown in FIG. 4A, the electron moves along the lowestpart of the oxide semiconductor film, which is energetically stable, atan interface between the gate insulating film and the highly purifiedoxide semiconductor film.

In FIG. 4B, when a negative potential (reverse bias) is applied to thegate electrode (GE1), a hole which is a minority carrier issubstantially zero; therefore, current is as close to zero as possible.

For example, even when the channel width W of the thin film transistoris 1×10⁴ μm and the channel length thereof is 3 μm, off-state current isless than or equal to 10⁻¹³ A, which is extremely small, and asubthreshold swing (S value) of 0.1 V/dec. (the gate insulating filmwith a thickness of 100 nm) is obtained.

As described above, the oxide semiconductor film is highly purified sothat impurities that are not main components of the oxide semiconductorfilm, typically hydrogen, water, hydroxyl groups, or hydride, may becontained as little as possible, whereby good operation of the thin filmtransistor can be obtained. In particular, off-state current can bereduced.

On the other hand, a horizontal thin film transistor in which a channelis formed substantially in parallel with a substrate needs a source anda drain as well as the channel, so that an area occupied by the thinfilm transistor in the substrate is increased, which hindersminiaturization. However, a source, a channel, and a drain are stackedin a vertical thin film transistor, whereby an occupation area in asubstrate surface can be reduced. As a result of this, it is possible tominiaturize the thin film transistor.

The channel length of the vertical thin film transistor can becontrolled by the thickness of the oxide semiconductor film; therefore,when the oxide semiconductor film 107 is formed to have a smallthickness, a thin film transistor having a short channel length can beprovided. When the channel length is reduced, series resistance of thesource, the channel, and the drain can be reduced; therefore, on-statecurrent and field-effect mobility of the thin film transistor can beincreased. The gate electrode of the thin film transistor described inthis embodiment has a ring shape, so that the channel width can beincreased, whereby an on-state current can be increased. In addition, athin film transistor having the highly purified oxide semiconductor filmwhose hydrogen concentration is reduced is in an insulating state whereoff-state current is extremely low and almost no current flows when thethin film transistor is off. Therefore, even when the thickness of theoxide semiconductor film is decreased to reduce the channel length ofthe vertical thin film transistor, a thin film transistor in whichalmost no off-state current in a non-conducting state flows can beprovided.

As described above, using a highly purified oxide semiconductor filmwhose hydrogen concentration is reduced makes it possible to manufacturea thin film transistor which is suitable for higher definition and hashigh operation speed and in which a large amount of current can flowwhen the thin film transistor is on and almost no current flows when thethin film transistor is off.

Embodiment 2

In this embodiment, a structure of a field-effect transistor, forexample a thin film transistor, which is different from that of the thinfilm transistor described in Embodiment 1, will be described withreference to FIGS. 6A and 6B.

FIG. 6A is a top view of thin film transistors 147 and 149, and FIG. 6Bcorresponds to a cross-sectional view taken along chain line A-B in FIG.6A.

As illustrated in FIG. 6B, the first electrode 105 and a first electrode106, the oxide semiconductor film 107, and the second electrode 109 arestacked over the insulating film 103 formed over the substrate 101. Thegate insulating film 111 is provided so as to cover the first electrodes105 and 106, the oxide semiconductor film 107, and the second electrode109. The third electrode 113 is provided over the gate insulating film111. The insulating film 117 functioning as an interlayer insulatingfilm is provided over the gate insulating film 111 and the thirdelectrode 113. Openings are formed in the insulating film 117. Thewiring 131 connected via the opening to the first electrode 105, awiring 132 connected via the opening to the first electrode 106 (seeFIG. 6A), the wiring 129 connected via the opening to the secondelectrode 109, and the wiring 125 connected via the opening to the thirdelectrode 113 are formed.

The first electrode 105 functions as one of a source electrode and adrain electrode of the thin film transistor 147. The first electrode 106functions as one of a source electrode and a drain electrode of the thinfilm transistor 149. The second electrode 109 functions as the other ofthe source electrode and the drain electrode of each of the thin filmtransistors 147 and 149. The third electrode 113 functions as a gateelectrode of each of the thin film transistors 147 and 149.

In this embodiment, the first electrode 105 and the first electrode 106are separated. Further, the thin film transistor 147 and the thin filmtransistor 149 are connected in series using the second electrode 109,the wiring 129, and the third electrode 113.

The thin film transistors 147 and 149 of this embodiment are formedusing a highly purified oxide semiconductor film whose hydrogenconcentration is reduced, in a manner similar to that of Embodiment 1.Therefore, good operation of the thin film transistor can be obtained.In particular, off-state current can be reduced. As a result of this, athin film transistor which is suitable for higher definition and hashigh operation speed and in which a large amount of current can flowwhen the thin film transistor is on and almost no current flows when thethin film transistor is off can be manufactured.

Embodiment 3

In this embodiment, a manufacturing process of the thin film transistorin FIGS. 1A and 1B will be described with reference to FIGS. 7A to 7E.

As illustrated in FIG. 7A, the insulating film 103 is formed over thesubstrate 101, and the first electrode 105 is formed over the insulatingfilm 103. The first electrode 105 functions as one of the sourceelectrode and the drain electrode of the thin film transistor.

The insulating film 103 can be formed by a sputtering method, a CVDmethod, a coating method, or the like.

Note that when the insulating film 103 is formed by a sputtering method,the insulating film 103 is preferably formed while hydrogen, water,hydroxyl groups, hydride, or the like remaining in a treatment chamberis removed. This is for preventing hydrogen, water, hydroxyl groups,hydride, or the like from being contained in the insulating film 103. Itis preferable to use an entrapment vacuum pump in order to removehydrogen, water, hydroxyl groups, hydride, or the like remaining in thetreatment chamber. As the entrapment vacuum pump, a cryopump, an ionpump, or a titanium sublimation pump is preferably used, for example.Further, as an evacuation unit, a cold trap may be added to a turbomolecular pump. Since impurities which are hydrogen, water, hydroxylgroups, hydride, or the like are removed from the treatment chamberwhich is evacuated using a cryopump, the insulating film 103 is formedin the treatment chamber when the concentration of impurities containedin the insulating film 103 can be reduced.

As a sputtering gas used for forming the insulating film 103, a highpurity gas is preferably used in which impurities such as hydrogen,water, hydroxyl groups, or hydride are reduced to such a level that theimpurity concentration is represented by the unit “ppm” or “ppb”.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used for a sputtering power source, aDC sputtering method in which a direct current power source is used, anda pulsed DC sputtering method in which a bias is applied in a pulsedmanner. The RF sputtering method is mainly used in the case where aninsulating film is formed, whereas the DC sputtering method is mainlyused in the case where a metal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or films of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

Alternatively, a sputtering apparatus provided with a magnet systeminside the chamber and used for a magnetron sputtering method, or asputtering apparatus used for an ECR sputtering method in which plasmagenerated with the use of microwaves is used without using glowdischarge can be used.

Further, as a deposition method using a sputtering method, a reactivesputtering method in which a target substance and a sputtering gascomponent are chemically reacted with each other during deposition toform a thin compound film thereof, or a bias sputtering method in whichvoltage is also applied to a substrate during deposition can be used.

As the sputtering in this specification, the above-described sputteringapparatus and the sputtering method can be employed as appropriate.

In this embodiment, the substrate 101 is transferred to the treatmentchamber. A sputtering gas containing high purity oxygen, from whichhydrogen, water, hydroxyl groups, hydride, or the like is removed, isintroduced into the treatment chamber, and a silicon oxide film isformed as the insulating film 103 over the substrate 101 using a silicontarget. Note that when the insulating film 103 is formed, the substrate101 may be heated.

For example, the silicon oxide film is formed by an RF sputtering methodunder the following conditions: quartz (preferably, synthesized quartz)is used; the substrate temperature is 108° C., the distance between thesubstrate and the target (the T-S distance) is 60 mm; the pressure is0.4 Pa; the high frequency power source is 1.5 kW; and the atmospherecontains oxygen and argon (oxygen flow rate of 25 sccm: argon flow rateof 25 sccm=1:1). The film thickness may be 100 nm. Note that instead ofquartz (preferably, synthesized quartz), a silicon target can be used.Note that as the sputtering gas, oxygen, or a mixed gas of oxygen andargon is used.

For example, when the insulating film 103 is formed using a stackedstructure, a silicon nitride film is formed using a silicon target and asputtering gas containing high purity nitrogen from which hydrogen,water, hydroxyl groups, hydride, or the like is removed, between thesilicon oxide film and the substrate. Also in this case, it ispreferable that a silicon nitride film be formed while hydrogen, water,hydroxyl groups, hydride, or the like remaining in the treatment chamberis removed in a manner similar to that of the silicon oxide film. Notethat in the process, the substrate 101 may be heated.

When a silicon nitride film and a silicon oxide film are stacked as theinsulating film 103, a silicon nitride film and a silicon oxide film canbe formed using a common silicon target in the same treatment chamber. Asputtering gas containing nitrogen is introduced into the treatmentchamber first, and a silicon nitride film is formed using a silicontarget provided in the treatment chamber; next, the sputtering gascontaining nitrogen is switched to a sputtering gas containing oxygenand a silicon oxide film is formed using the same silicon target. Thesilicon nitride film and the silicon oxide film can be formed insuccession without being exposed to air; therefore, impurities such ashydrogen, water, hydroxyl groups, or hydride can be prevented from beingadsorbed on the surface of the silicon nitride film.

The first electrode 105 can be formed in such a manner that a conductivefilm is formed over the insulating film 103 by a sputtering method, aCVD method, or a vacuum evaporation method, a resist mask is formed overthe conductive film in a photolithography step, and the conductive filmis etched using the resist mask. Alternatively, the first electrode 105is formed by a printing method or an ink-jet method without using aphotolithography step, so that the number of steps can be reduced. Notethat end portions of the first electrode 105 preferably have a taperedshape, so that the coverage with a gate insulating film to be formedlater improves. When the angle formed between the end portion of thefirst electrode 105 and the insulating film 103 is greater than or equalto 30° and less than or equal to 60°, preferably greater than or equalto 40° and less than or equal to 50°, the coverage with the gateinsulating film to be formed later can be improved.

In this embodiment, as the conductive film to serve as the firstelectrode 105, a titanium film is formed to have a thickness of 50 nm bya sputtering method, an aluminum film is formed to have a thickness of100 nm, and a titanium film is formed to have a thickness of 50 nm.Next, etching is performed using the resist mask formed in thephotolithography step, whereby the island-shaped first electrode 105 isformed.

Next, as illustrated in FIG. 7B, the oxide semiconductor film 107 andthe second electrode 109 are formed over the first electrode 105. Theoxide semiconductor film 107 functions as a channel formation region ofthe thin film transistor, and the second electrode 109 functions as theother of the source electrode or the drain electrode of the thin filmtransistor.

Here, a method for manufacturing the oxide semiconductor film 107 andthe second electrode 109 will be described.

An oxide semiconductor film is formed by a sputtering method over thesubstrate 101 and the first electrode 105. Next, a conductive film isformed over the oxide semiconductor film.

As pretreatment, it is preferable that the substrate 101 provided withthe first electrode 105 be preheated in a preheating chamber of asputtering apparatus and impurities such as hydrogen, water, hydroxylgroups, or hydride adsorbed on the substrate 101 be eliminated andremoved so that hydrogen may be contained in the oxide semiconductorfilm 107 as little as possible. Note that a cryopump is preferable foran evacuation unit provided in the preheating chamber. Note that thispreheating treatment can be omitted. In addition, this preheating may beperformed on the substrate 101 before the formation of the gateinsulating film 111, or may be performed on the substrate 101 before theformation of the third electrode 113.

Note that before the oxide semiconductor film is formed by a sputteringmethod, reverse sputtering in which plasma is generated by introductionof an argon gas is preferably performed to clean the surface of thefirst electrode 105, so that resistance at the interface between thefirst electrode 105 and the oxide semiconductor film can be reduced. Thereverse sputtering refers to a method in which, without application ofvoltage to a target side, a high-frequency power source is used forapplication of voltage to a substrate side in an argon atmosphere togenerate plasma in the vicinity of the substrate to modify a surface.Note that instead of an argon atmosphere, a nitrogen atmosphere, ahelium atmosphere, or the like may be used.

In this embodiment, the oxide semiconductor film is formed by asputtering method with the use of an In—Ga—Zn—O-based oxidesemiconductor target for film formation. Alternatively, the oxidesemiconductor film can be formed by a sputtering method in a rare gas(typically argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere of a rare gas (typically argon) and oxygen. When a sputteringmethod is employed, a target containing SiO₂ at greater than or equal to2 wt % and less than or equal to 10 wt % may be used.

As a sputtering gas used for forming the oxide semiconductor film, ahigh purity gas is preferably used in which impurities such as hydrogen,water, hydroxyl groups, or hydride are reduced to such a level that theimpurity concentration is represented by the unit “ppm” or “ppb”.

As a target used to form the oxide semiconductor film by a sputteringmethod, a target of a metal oxide containing zinc oxide as a maincomponent can be used. As another example of a target of a metal oxide,an oxide semiconductor target for film formation containing In, Ga, andZn (in a composition ratio, In₂O₃:Ga₂O₃:ZnO=1:1:1 [mol %],In:Ga:Zn=1:1:0.5 [atom %]) can be used. Alternatively, as an oxidesemiconductor target for film formation containing In, Ga, and Zn, atarget having such a composition ratio that In:Ga:Zn=1:1:1 [atom %] orIn:Ga:Zn=1:1:2 [atom %] can be used. The filling rate of the oxidesemiconductor target for film formation is greater than or equal to 90%and less than or equal to 100%, preferably, greater than or equal to 95%and less than or equal to 99.9%. An oxide semiconductor film formedusing the oxide semiconductor target for film formation with highfilling rate is dense.

The oxide semiconductor film is formed over the insulating film 103 andthe second electrode 109 in such a manner that a sputtering gas fromwhich hydrogen, water, hydroxyl groups, hydride, or the like is removedis introduced into the treatment chamber and a metal oxide is used as atarget while the substrate is held in the treatment chamber held in areduced-pressure state and moisture remaining in the treatment chamberis removed. It is preferable to use an entrapment vacuum pump in orderto remove hydrogen, water, hydroxyl groups, hydride, or the likeremaining in the treatment chamber. A cryopump, an ion pump, or atitanium sublimation pump is preferably used, for example. Further, asan evacuation unit, a cold trap may be added to a turbo molecular pump.For example, hydrogen, water, hydroxyl groups, hydride, or the like(more preferably, also a compound containing a carbon atom) are removedfrom the treatment chamber which is evacuated using a cryopump;therefore, the concentration of impurities contained in the oxidesemiconductor film can be reduced. The oxide semiconductor film may beformed while the substrate is heated.

In this embodiment, as an example of a film formation condition of theoxide semiconductor film, the following conditions are applied: thesubstrate temperature is room temperature, the distance between thesubstrate and the target is 110 mm; the pressure is 0.4 Pa; the directcurrent (DC) power source is 0.5 kW; and the atmosphere contains oxygenand argon (oxygen flow rate of 15 sccm: argon flow rate of 30 sccm).Note that a pulsed direct current (DC) power source is preferablebecause powder substances (also referred to as particles or dust)generated in film formation can be reduced and the film thickness can beuniform. The oxide semiconductor film preferably has a thickness ofgreater than or equal to 30 nm and less than or equal to 3000 nm. Notethat the appropriate thickness is different according to a material usedfor the oxide semiconductor film, and the thickness may be selected asappropriate in accordance with a material.

As the sputtering method and the sputtering apparatus that are used whenthe oxide semiconductor film is formed, the sputtering method and thesputtering apparatus which are employed for the insulating film 103 canbe used as appropriate.

The conductive film to serve as the second electrode 109 can be formedusing the material and the method which are used for the first electrode105, as appropriate. Here, as the conductive film to serve as the secondelectrode 109, a 50-nm-thick titanium film, a 100-nm-thick aluminumfilm, and a 50-nm-thick titanium film are stacked in that order.

Next, a resist mask is formed over the conductive film in aphotolithography step, the conductive film to serve as the secondelectrode 109 and the oxide semiconductor film to serve as the oxidesemiconductor film 107 are etched using the resist mask, whereby theisland-shaped second electrode 109 and the island-shaped oxidesemiconductor film 107 are formed. Instead of the resist mask formed inthe photolithography step, a resist mask is formed using an ink-jetmethod, so that the number of steps can be reduced. When the angleformed between the first electrode 105 and the end portions of thesecond electrode 109 and the oxide semiconductor film 107 is greaterthan or equal to 30° and less than or equal to 60°, preferably greaterthan or equal to 40° and less than or equal to 50° because of theetching, the coverage with a gate insulating film to be formed later canbe improved.

Note that the etching of the conductive film and the oxide semiconductorfilm here may be performed using either dry etching or wet etching, orusing both dry etching and wet etching. In order to form the oxidesemiconductor film 107 and the second electrode 109 each having adesired shape, an etching condition (etchant, etching time, temperature,or the like) is adjusted as appropriate in accordance with a material.

When the etching rate of each of the conductive film to serve as thesecond electrode 109 and the oxide semiconductor film is different fromthat of the first electrode 105, a condition such that the etching rateof the first electrode 105 is low and the etching rate of each of theconductive film to serve as the second electrode 109 and the oxidesemiconductor film is high is selected. Alternatively, when a conditionsuch that the etching rate of the oxide semiconductor film is low andthe etching rate of the conductive film to serve as the second electrode109 is high is selected, the conductive film to serve as the secondelectrode 109 is etched; then, a condition such that the etching rate ofthe first electrode 105 is low and the etching rate of the oxidesemiconductor film is high is selected.

As an etchant used for wet etching the oxide semiconductor film, a mixedsolution of phosphoric acid, acetic acid, and nitric acid, an ammoniahydrogen peroxide mixture (hydrogen peroxide: ammonia: water=5:2:2), orthe like can be used. In addition, ITO07N (produced by KANTO CHEMICALCO., INC.) may also be used.

The etchant after the wet etching is removed together with the etchedmaterials by cleaning. The waste liquid containing the etchant and thematerial etched off may be purified and the material may be reused. Whena material such as indium contained in the oxide semiconductor film iscollected from the waste liquid after the etching and reused, theresources can be efficiently used and the cost can be reduced.

As an etching gas used for dry etching the oxide semiconductor film, agas containing chlorine (a chlorine-based gas such as chlorine (Cl₂),boron trichloride (BCl₃), silicon tetrachloride (SiCl₄), or carbontetrachloride (CCl₄)) is preferably used.

Alternatively, a gas containing fluorine (fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogen fluoride(NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr); oxygen (O₂);any of these gases to which a rare gas such as helium (He) or argon (Ar)is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. In order to etch the film into a desired shape, the etchingcondition (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

In this embodiment, the conductive film to serve as the second electrode109 is etched using an ammonia hydrogen peroxide mixture (a mixture ofammonia, water, and hydrogen peroxide water) as an etchant, and then theoxide semiconductor film is etched using a solution in which phosphoricacid, acetic acid, and nitric acid are mixed, whereby the oxidesemiconductor film 107 is formed.

Next, in this embodiment, first heat treatment is performed. The firstheat treatment is performed at a temperature higher than or equal to400° C. and lower than or equal to 750° C., preferably, higher than orequal to 400° C. and lower than a strain point of the substrate. Here,the substrate is introduced into an electric furnace which is one ofheat treatment apparatuses, and heat treatment is performed on the oxidesemiconductor film in an inert gas atmosphere, such as a nitrogenatmosphere or a rare gas atmosphere, at 450° C. for one hour, and thenthe oxide semiconductor film is not exposed to air. Accordingly,hydrogen, water, hydroxyl groups, hydride, or the like can be preventedfrom being mixed into the oxide semiconductor film, hydrogenconcentration is reduced, and the oxide semiconductor film is highlypurified, whereby an i-type oxide semiconductor film or a substantiallyi-type oxide semiconductor film can be obtained. That is, at least oneof dehydration and dehydrogenation of the oxide semiconductor film 107can be performed by this first heat treatment.

Note that it is preferable that in the first heat treatment, hydrogen,water, hydroxyl groups, hydride, or the like be not contained innitrogen or a rare gas such as helium, neon, or argon. Alternatively,the purity of nitrogen or a rare gas such as helium, neon, or argonintroduced into a heat treatment apparatus is preferably higher than orequal to 6N (99.9999%), more preferably higher than or equal to 7N(99.99999%) (that is, the concentration of the impurities is 1 ppm orlower, preferably 0.1 ppm or lower).

In accordance with conditions of the first heat treatment or a materialfor the oxide semiconductor film, the oxide semiconductor film iscrystallized and changed to a microcrystalline film or a polycrystallinefilm in some cases. For instance, the oxide semiconductor film may becrystallized to be a microcrystalline oxide semiconductor film having adegree of crystallization of greater than or equal to 90%, or greaterthan or equal to 80%. Further, depending on the conditions of the firstheat treatment and the material for the oxide semiconductor film, theoxide semiconductor film may become amorphous oxide semiconductor filmcontaining no crystalline component. The oxide semiconductor film maybecome an oxide semiconductor film in which a microcrystalline portion(with a grain diameter greater than or equal to 1 nm and less than orequal to 20 nm, typically greater than or equal to 2 nm and less than orequal to 4 nm) is mixed into the amorphous oxide semiconductor film.

Alternatively, the first heat treatment of the oxide semiconductor filmmay be performed on the oxide semiconductor film before theisland-shaped oxide semiconductor film is formed. In that case, thesubstrate is taken out from the heat apparatus after the first heattreatment, and then a photolithography step is performed.

Note that the heat treatment which has an effect of dehydration ordehydrogenation on the oxide semiconductor film may be performed afterthe oxide semiconductor film is formed; after the conductive film toserve as the second electrode is stacked over the oxide semiconductorfilm; after the gate insulating film is formed over the first electrode,the oxide semiconductor film, and the second electrode; or after thegate electrode is formed.

Next, as illustrated in FIG. 7C, the gate insulating film 111 is formedover the first electrode 105, the oxide semiconductor film 107, and thesecond electrode 109.

The i-type oxide semiconductor film (the highly purified oxidesemiconductor film whose hydrogen concentration is reduced) or thesubstantially i-type oxide semiconductor film because of the removal ofimpurities is extremely sensitive to an interface state and interfacecharge; therefore, the interface between the oxide semiconductor filmand the gate insulating film 111 is important. Accordingly, the gateinsulating film 111 which is in contact with the highly purified oxidesemiconductor film needs high quality.

For example, a high-quality insulating film which is dense and which hashigh withstand voltage can be formed by high density plasma CVD usingmicrowaves (2.45 GHz), which is preferable. This is because when thehighly purified oxide semiconductor film whose hydrogen concentration isreduced and the high-quality gate insulating film are close to eachother, the interface state can be reduced and the interfacecharacteristics can be made favorable.

Needless to say, other film formation methods, such as a sputteringmethod or a plasma CVD method, can be applied as long as a high-qualityinsulating film can be formed as the gate insulating film. A gateinsulating film whose film quality is improved by the heat treatmentafter the gate insulating film is formed, or an insulating film whosecharacteristics of an interface with the oxide semiconductor film areimproved may be used. In any case, not to mention good film quality asthe gate insulating film, a gate insulating film in which the interfacestate density with the oxide semiconductor film can be reduced and agood interface can be formed may be used.

In a gate bias-temperature stress test (BT test) at 85° C., at a voltageapplied to the gate of 2×10⁶ V/cm for 12 hours, when impurities areadded to the oxide semiconductor film, bonds between impurities and amain component of the oxide semiconductor film are cut by an intenseelectric field (B: bias) and high temperature (T: temperature), andgenerated dangling bonds cause a drift of threshold voltage (Vth).

On the other hand, impurities of the oxide semiconductor film, inparticular, hydrogen, water, or the like, are removed as much aspossible, and characteristics of an interface between the oxidesemiconductor film and the gate insulating film are good as describedabove, whereby a thin film transistor which is stable to the BT test canbe obtained.

When the gate insulating film 111 is formed by a sputtering method, thehydrogen concentration in the gate insulating film 111 can be reduced.When a silicon oxide film is formed by a sputtering method, a silicontarget or a quartz target is used as a target and oxygen or a mixed gasof oxygen and argon is used as a sputtering gas.

Note that a halogen element (e.g. fluorine or chlorine) is contained inan insulating film provided in contact with the oxide semiconductorfilm, or a halogen element is contained in an oxide semiconductor filmby plasma treatment in a gas atmosphere containing a halogen element ina state that the oxide semiconductor film is exposed, whereby impuritiessuch as hydrogen, water, hydroxyl groups, or hydride (also referred toas hydrogen compound) existing in the oxide semiconductor film or at theinterface between the oxide semiconductor film and the insulating filmwhich is provided in contact with the oxide semiconductor film may beremoved. When the insulating film contains a halogen element, thehalogen element concentration in the insulating film may beapproximately 5×10¹⁸ atoms/cm³ to 1×10²⁰ atoms/cm³.

As described above, in the case where a halogen element is contained inthe oxide semiconductor film or at the interface between the oxidesemiconductor film and the insulating film which is in contact with theoxide semiconductor film and the insulating film which is provided incontact with the oxide semiconductor film is an oxide insulating film,the oxide insulating film on the side where the oxide semiconductor filmis not in contact with the oxide insulating film is preferably coveredwith a nitrogen insulating film. That is, a silicon nitride film or thelike may be provided on and in contact with the oxide insulating filmwhich is on and in contact with the oxide semiconductor film. With sucha structure, impurities such as hydrogen, water, hydroxyl groups, orhydride can be prevented from entering the oxide insulating film.

The gate insulating film 111 can have a structure in which a siliconoxide film and a silicon nitride film are stacked in that order over thefirst electrode 105, the oxide semiconductor film 107, and the secondelectrode 109. For example, a silicon oxide film (SiO_(x) (x>0)) havinga thickness of greater than or equal to 5 nm and less than or equal to300 nm is formed as the first gate insulating film, and a siliconnitride film (SiN_(y) (y>0)) having a thickness of greater than or equalto 50 nm and less than or equal to 200 nm is stacked as the second gateinsulating film over the first gate insulating film by a sputteringmethod, so that a gate insulating film having a thickness of 100 nm maybe formed. In this embodiment, the silicon oxide film having a thicknessof 100 nm is formed by an RF sputtering method under the followingconditions: the pressure is 0.4 Pa; the high-frequency power source is1.5 kW; and the atmosphere contains oxygen and argon (oxygen flow rateof 25 sccm: argon flow rate of 25 sccm=1:1).

Next, second heat treatment may be performed in an inert gas atmosphereor an oxygen gas atmosphere (preferably, at a temperature higher than orequal to 200° C. and lower than or equal to 400° C., for example, atemperature higher than or equal to 250° C. and lower than or equal to350° C.). Note that the second heat treatment may be performed after theformation of any one of the third electrode 113, the insulating film117, and the wirings 125 and 129, which is performed later. Hydrogen ormoisture contained in the oxide semiconductor film can be diffused intothe gate insulating film by the heat treatment.

Then, the third electrode 113 functioning as a gate electrode is formedover the gate insulating film 111.

The third electrode 113 can be formed in such a way that a conductivefilm to serve as the third electrode 113 is formed over the gateinsulating film 111 by a sputtering method, a CVD method, or a vacuumevaporation method, a resist mask is formed in a photolithography stepover the conductive film, and the conductive film is etched using theresist mask.

In this embodiment, after a titanium film having a thickness of 150 nmis formed by a sputtering method, etching is performed using a resistmask formed in a photolithography step, so that the third electrode 113is formed.

Through the above process, the thin film transistor 145 having thehighly purified oxide semiconductor film 107 whose hydrogenconcentration is reduced can be formed.

Next, as illustrated in FIG. 7D, after the insulating film 117 is formedover the gate insulating film 111 and the third electrode 113, contactholes 119 and 123 are formed.

The insulating film 117 is formed using an oxide insulating film such asa silicon oxide film, a silicon oxynitride film, an aluminum oxide film,or an aluminum oxynitride film; or a nitride insulating film such as asilicon nitride film, a silicon nitride oxide film, an aluminum nitridefilm, or an aluminum nitride oxide film. Alternatively, an oxideinsulating film and a nitride insulating film can be stacked.

The insulating film 117 is formed by a sputtering method, a CVD method,or the like. Note that when the insulating film 117 is formed by asputtering method, the substrate 101 is heated to a temperature of 100°C. to 400° C., a sputtering gas in which hydrogen, water, hydroxylgroups, hydride, or the like is removed and which contains high puritynitrogen is introduced, and an insulating film may be formed using asilicon target. Also in this case, an insulating film is preferablyformed while hydrogen, water, hydroxyl groups, hydride, or the likeremaining in the treatment chamber is removed.

Note that after the insulating film 117 is formed, heat treatment may beperformed in the atmosphere at a temperature higher than or equal to100° C. and lower than or equal to 200° C. for greater than or equal to1 hour and less than or equal to 30 hours. A normally-off thin filmtransistor can be obtained by this heat treatment. Therefore,reliability of the display device and the semiconductor device can beimproved.

A resist mask is formed in a photolithography step, and parts of thegate insulating film 111 and the insulating film 117 are removed byselective etching, whereby the contact holes 119 and 123 which reach thefirst electrode 105, the second electrode 109, and the third electrode113 are formed.

Next, after a conductive film is formed over the gate insulating film111 and the contact holes 119 and 123, etching is performed using aresist mask formed in a photolithography step, whereby the wirings 125and 129 are formed. Note that a resist mask may be formed by an ink-jetmethod. No photomask is used when a resist mask is formed by an ink-jetmethod; therefore, production cost can be reduced.

The wirings 125 and 129 can be formed in a manner similar to that of thefirst electrode 105.

Note that a planarization insulating film for planarization may beprovided between the third electrode 113 and the wiring 125. An organicmaterial having heat resistance, such as polyimide, acrylic,benzocyclobutene, polyamide, or epoxy can be used as typical examples ofthe planarization insulating film. Other than such organic materials, itis also possible to use a low-dielectric constant material (a low-kmaterial), a siloxane-based resin, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), or the like. Note that theplanarization insulating film may be formed by stacking a plurality ofinsulating films formed from these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include an organic group (e.g. analkyl group or an aryl group) or a fluoro group as a substituent.Moreover, the organic group may include a fluoro group.

There is no particular limitation on the method for forming theplanarization insulating film. The planarization insulating film can beformed, depending on the material, by a method such as a sputteringmethod, an SOG method, a spin coating method, a dipping method, a spraycoating method, or a droplet discharge method (e.g. an ink-jet method,screen printing, or offset printing), or a tool such as a doctor knife,a roll coater, a curtain coater, or a knife coater.

As described above, the hydrogen concentration in the oxidesemiconductor film can be reduced, and the oxide semiconductor film canbe highly purified. Accordingly, the oxide semiconductor film can bestabilized. In addition, an oxide semiconductor film which has anextremely small number of minority carriers and a wide band gap can beformed by heat treatment at a temperature of lower than or equal to theglass transition temperature. Therefore, a thin film transistor can beformed using a large-area substrate; thus, the mass productivity can beimproved. In addition, using a highly purified oxide semiconductor filmwhose hydrogen concentration is reduced makes it possible to manufacturea thin film transistor which is suitable for higher definition and hashigh operation speed and in which a large amount of current can flowwhen the thin film transistor is on and almost no current flows when thethin film transistor is off.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 4

In this embodiment, a field-effect transistor, for example a thin filmtransistor in which an oxide semiconductor film that is different fromthe oxide semiconductor film described in Embodiment 3 is included willbe described with reference to FIGS. 7A and 7B and FIGS. 8A and 8B.

In a manner similar to that in Embodiment 3, as illustrated in FIG. 7A,the insulating film 103 and the first electrode 105 are formed over thesubstrate 101. Next, as illustrated in FIG. 7B, the oxide semiconductorfilm 107 and the second electrode 109 are formed over the firstelectrode 105.

Next, the first heat treatment is performed. The first heat treatment inthis embodiment is different from the first heat treatment in the aboveembodiment. The heat treatment makes it possible to form the crystalgrains in the surface of an oxide semiconductor film 151 as illustratedin FIG. 8B. In this embodiment, the first heat treatment is performedwith an apparatus for heating an object to be processed by at least oneof thermal conduction and thermal radiation from a heater such as aresistance heater. Here, the temperature of the heat treatment is higherthan or equal to 500° C. and lower than or equal to 700° C., preferablyhigher than or equal to 650° C. and lower than or equal to 700° C. Notethat, although there is no requirement for the upper limit of the heattreatment temperature from the essential part of the invention, theupper limit of the heat treatment temperature needs to be within theallowable temperature limit of the substrate 101. In addition, thelength of time of the heat treatment is preferably greater than or equalto 1 minute and less than or equal to 10 minutes. When RTA treatment isemployed for the first heat treatment, the heat treatment can beperformed in a short time; thus, adverse effects of heat on thesubstrate 101 can be reduced. In other words, the upper limit of theheat treatment temperature can be raised in this case as compared withthe case where heat treatment is performed for a long time. In addition,the crystal grains having predetermined structures can be selectivelyformed in the vicinity of the surface of the oxide semiconductor film.

As examples of the heat treatment apparatus that can be used in thisembodiment, rapid thermal anneal (RTA) apparatuses such as a gas rapidthermal anneal (GRTA) apparatus and a lamp rapid thermal anneal (LRTA)apparatus, and the like are given. An LRTA apparatus is an apparatus forheating an object to be processed by radiation of light (anelectromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As the gas,an inert gas which does not react with an object to be processed by heattreatment, such as nitrogen or a rare gas such as argon is used.

For example, as the first heat treatment, GRTA may be performed in whichthe substrate is moved into an atmosphere of an inert gas such asnitrogen or a rare gas which has been heated to a temperature as high as650° C. to 700° C.; heated for several minutes; and moved out of thehighly-heated inert gas. GRTA enables high-temperature heat treatment tobe performed in a short time.

Note that in the first heat treatment, it is preferable that hydrogen,water, hydroxyl groups, hydride, or the like be not contained innitrogen or a rare gas such as helium, neon, or argon. Alternatively,the purity of nitrogen or a rare gas such as helium, neon, or argon thatis introduced into the heat treatment apparatus is preferably 6N(99.9999%) or higher, more preferably 7N (99.99999%) or higher (that is,the impurity concentration is 1 ppm or lower, preferably 0.1 ppm orlower).

Note that the above heat treatment may be performed at any timing aslong as it is performed after the oxide semiconductor film 107 isformed; however, in order to promote dehydration or dehydrogenation, theheat treatment is preferably performed before other components areformed on a surface of the oxide semiconductor film 107. In addition,the heat treatment may be performed plural times instead of once.

FIG. 8B is an enlarged view of a dashed line portion 153 in FIG. 8A.

The oxide semiconductor film 151 includes an amorphous region 155 thatmainly contains an amorphous oxide semiconductor and crystal grains 157that are formed in the surface of the oxide semiconductor film 151.Further, the crystal grains 157 are formed in a region that is locatedat a distance (depth) from the surface of the oxide semiconductor film151 of 20 nm or less (i.e., in the vicinity of the surface). Note thatthe location where the crystal grains 157 are formed is not limited tothe above in the case where the thickness of the oxide semiconductorfilm 151 is large. For example, in the case where the oxidesemiconductor film 151 has a thickness of 200 nm or more, the “vicinityof a surface (surface vicinity)” means a region that is located at adistance (depth) from the surface of 10% or less of the thickness of theoxide semiconductor film.

Here, the amorphous region 155 mainly contains an amorphous oxidesemiconductor film. Note that the word “mainly” means, for example, astate where one occupies 50% or more of a region. In this case, it meansa state where the amorphous oxide semiconductor film occupies 50% ormore at volume % (or weight %) of the amorphous region 155. In otherwords, the amorphous region in some cases includes crystals of an oxidesemiconductor film other than an amorphous oxide semiconductor film, andthe percentage of the content thereof is preferably less than 50% atvolume % (or weight %). However, the percentage of the content is notlimited to the above.

In the case where an In—Ga—Zn—O-based oxide semiconductor is used as amaterial for the oxide semiconductor film, the composition of the aboveamorphous region 155 is preferably set so that the Zn content (atomic %)is less than the In or Ga content (atomic %) for the reason that suchcomposition makes it easy for the crystal grains 157 which havepredetermined composition to be formed.

After that, a gate insulating film and a third electrode that functionsas a gate electrode are formed in a manner similar to that of Embodiment3 to complete the thin film transistor.

The vicinity of the surface of the oxide semiconductor film 151, whichis in contact with the gate insulating film, serves as a channel. Thecrystal grains are included in the region that serves as a channel,whereby the resistance between a source, the channel, and a drain isreduced and carrier mobility is increased. Thus, the field-effectmobility of the thin film transistor in which the oxide semiconductorfilm 151 is included is increased, which leads to favorable electriccharacteristics of the thin film transistor.

Further, the crystal grains 157 are more stable than the amorphousregion 155; thus, when the crystal grains 157 are included in thevicinity of the surface of the oxide semiconductor film 151, entry ofimpurities (e.g., hydrogen, water, hydroxyl groups, or hydride) into theamorphous region 155 can be reduced. Thus, the reliability of the oxidesemiconductor film 151 can be improved.

Through the above-described steps, the concentration of hydrogen in theoxide semiconductor film can be reduced and the oxide semiconductor filmis highly purified. Thus, stabilization of the oxide semiconductor filmcan be achieved. In addition, heat treatment at a temperature of lowerthan or equal to the glass transition temperature makes it possible toform an oxide semiconductor film with a wide band gap in which thenumber of minority carriers is extremely small. Thus, thin filmtransistors can be manufactured using a large substrate, which leads tothe enhancement of mass production. Further, the use of the oxidesemiconductor film in which the hydrogen concentration is reduced andthe purity is increased makes it possible to manufacture a thin filmtransistor which is suitable for increase in definition and has highoperation speed and in which a large amount of current can flow when thethin film transistor is on and almost no current flows when the thinfilm transistor is off.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 5

In this embodiment, a manufacturing process of the thin film transistorillustrated in FIGS. 1A and 1B will be described with reference to FIGS.7A to 7E.

In a manner similar to that of Embodiment 3, as illustrated in FIG. 7A,the first electrode 105 is formed over the substrate 101.

Next, as illustrated in FIG. 7B, the oxide semiconductor film 107 andthe second electrode 109 are formed over the first electrode 105.

Note that before the oxide semiconductor film is formed by a sputteringmethod, reverse sputtering in which plasma is generated by introductionof an argon gas is preferably performed so that dust or an oxide filmwhich is attached to a surface of the first electrode 105 is removed, inwhich case the resistance at an interface between the first electrode105 and the oxide semiconductor film can be reduced. Note that insteadof an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, orthe like may be used.

The oxide semiconductor film is formed over the substrate 101 and thefirst electrode 105 by a sputtering method. Then, a conductive film isformed over the oxide semiconductor film.

In this embodiment, the oxide semiconductor film is formed by asputtering method using an In—Ga—Zn—O-based oxide semiconductor targetfor formation. In this embodiment, the substrate is held in a treatmentchamber which is maintained in a reduced pressure state, and thesubstrate is heated to room temperature or a temperature lower than 400°C. Then, a sputtering gas from which hydrogen, water, hydroxyl groups,hydride, or the like is removed is introduced while hydrogen, water,hydroxyl groups, hydride, or the like remaining in the treatment chamberis removed, whereby the oxide semiconductor film is formed over thesubstrate 101 and the first electrode 105. An entrapment vacuum pump ispreferably used for removing hydrogen, water, hydroxyl groups, hydride,or the like remaining in the treatment chamber. For example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. Anevacuation unit may be a turbo pump provided with a cold trap. In thetreatment chamber evacuated with a cryopump, for example, hydrogen,water, hydroxyl groups, hydride (more preferably a compound containing acarbon atom), or the like is eliminated; thus, the concentration ofimpurities contained in the oxide semiconductor film formed in thetreatment chamber can be reduced. Further, sputtering formation isperformed while hydrogen, water, hydroxyl groups, hydride, or the likeremaining in the treatment chamber is removed with a cryopump, wherebyan oxide semiconductor film in which impurities such as hydrogen atomsand water are reduced can be formed even at a substrate temperature ofroom temperature to a temperature lower than 400° C.

In this embodiment, film formation conditions that the distance betweenthe substrate and the target is 100 mm, the pressure is 0.6 Pa, thedirect-current (DC) power supply is 0.5 kW, and the atmosphere is anoxygen atmosphere (the proportion of oxygen flow is 100%) are employed.Note that a pulsed direct-current (DC) power supply is preferably used,in which case powdery substances (also referred to as particles or dust)which are generated at the time of film formation can be reduced and thefilm thickness can be uniform. The oxide semiconductor film preferablyhas a thickness of greater than or equal to 30 nm and less than or equalto 3000 nm. Note that the appropriate thickness of the oxidesemiconductor film differs depending on the material to be used;therefore, the thickness may be determined as appropriate in accordancewith the material.

Note that the sputtering method and sputtering apparatus that are usedfor forming the insulating film 103 can be used as appropriate as asputtering method and a sputtering apparatus for forming the oxidesemiconductor film.

Next, a conductive film that serves as the second electrode 109 isformed using the material and method that are used for forming the firstelectrode 105.

Next, in a manner similar to that of Embodiment 3, the conductive filmthat serves as the second electrode 109 and the oxide semiconductor filmthat serves as the oxide semiconductor film 107 are etched so that theisland-shaped second electrode 109 and the island-shaped oxidesemiconductor film 107 are formed. The etching conditions (such as anetchant, etching time, and temperature) are adjusted as appropriate inaccordance with the material in order to form the oxide semiconductorfilm 107 and the second electrode 109 with desired shapes.

Next, as illustrated in FIG. 7C, in a manner similar to that ofEmbodiment 3, the gate insulating film 111 is formed over the firstelectrode 105, the oxide semiconductor film 107, and the secondelectrode 109. As the gate insulating film 111, a gate insulating filmthat has a favorable characteristic of an interface between the gateinsulating film 111 and the oxide semiconductor film 107 is preferable.The gate insulating film 111 is preferably formed by high-density plasmaCVD method using a microwave (2.45 GHz), in which case the gateinsulating film 111 can be dense and can have high withstand voltage andhigh quality. Another method such as a sputtering method or a plasma CVDmethod can be employed as long as the method enables a good-qualityinsulating film to be formed as the gate insulating film.

Note that before the gate insulating film 111 is formed, reversesputtering is preferably performed so that resist residues and the likeattached to at least a surface of the oxide semiconductor film 107 areremoved.

Further, before the gate insulating film 111 is formed, hydrogen, water,hydroxyl groups, hydride, or the like attached to an exposed surface ofthe oxide semiconductor film may be removed by plasma treatment using agas such as N₂O, N₂, or Ar. Alternatively, plasma treatment may beperformed using a mixed gas of oxygen and argon. In the case whereplasma treatment is performed, the gate insulating film 111 which is tobe in contact with part of the oxide semiconductor film is preferablyformed without being exposed to air.

Further, it is preferable that the substrate 101 over which componentsup to and including the first electrode 105 to the second electrode 109are formed be preheated in a preheating chamber in a sputteringapparatus as pretreatment to eliminate and evacuate hydrogen, water,hydroxyl groups, hydride, or the like adsorbed on the substrate 101 sothat hydrogen, water, hydroxyl groups, hydride, or the like is containedas little as possible in the gate insulating film 111. Alternatively, itis preferable that the substrate 101 be preheated in a preheatingchamber in a sputtering apparatus to eliminate and evacuate impuritiessuch as hydrogen, water, hydroxyl groups, hydride, or the like adsorbedon the substrate 101 after the gate insulating film 111 is formed. Notethat the temperature of the preheating is higher than or equal to 100°C. and lower than or equal to 400° C., preferably higher than or equalto 150° C. and lower than or equal to 300° C. A cryopump is preferableas an evacuation unit provided in the preheating chamber. Note that thispreheating treatment can be omitted.

The gate insulating film 111 can have a structure in which a siliconoxide film and a silicon nitride film are stacked in that order over thefirst electrode 105, the oxide semiconductor film 107, and the secondelectrode 109. For example, a silicon oxide film (SiO_(x) (x>0)) with athickness of greater than or equal to 5 nm and less than or equal to 300nm is formed as a first gate insulating film by a sputtering method anda silicon nitride film (SiN_(y) (y>0)) with a thickness of greater thanor equal to 50 nm and less than or equal to 200 nm is stacked as asecond gate insulating film over the first gate insulating film, wherebythe gate insulating film 111 is formed.

Next, as illustrated in FIG. 7C, in a manner similar to that ofEmbodiment 3, the third electrode 113 that functions as a gate electrodeis formed over the gate insulating film 111.

Through the above-described steps, the thin film transistor 145 in whichthe oxide semiconductor film 107 in which the hydrogen concentration isreduced is included can be manufactured.

Hydrogen, water, hydroxyl groups, hydride, or the like remaining in areaction atmosphere is removed in forming the oxide semiconductor filmas described above, whereby the concentration of hydrogen in the oxidesemiconductor film can be reduced. Thus, stabilization of the oxidesemiconductor film can be achieved.

Next, as illustrated in FIG. 7D, in a manner similar to that ofEmbodiment 3, the contact holes 119 and 123 are formed after theinsulating film 117 is formed over the gate insulating film 111 and thethird electrode 113.

Next, as illustrated in FIG. 7E, in a manner similar to that ofEmbodiment 3, the wirings 125 and 129 are formed.

Note that in a manner similar to that of Embodiment 3, after theformation of the insulating film 117, heat treatment may be furtherperformed at a temperature higher than or equal to 100° C. and lowerthan or equal to 200° C. in air for greater than or equal to 1 hour andless than or equal to 30 hours. This heat treatment enables anormally-off thin film transistor to be obtained. Thus, the reliabilityof a display device or a semiconductor device can be improved.

Note that a planarization insulating film for planarization may beprovided between the third electrode 113 and the wirings 125 and 129.

Hydrogen, water, hydroxyl groups, hydride, or the like remaining in areaction atmosphere is removed in forming the oxide semiconductor filmas described above, whereby the concentration of hydrogen in the oxidesemiconductor film can be reduced and the purity of the oxidesemiconductor film can be increased. Thus, stabilization of the oxidesemiconductor film can be achieved. In addition, heat treatment at atemperature lower than or equal to the glass transition temperaturemakes it possible to form an oxide semiconductor film with a wide bandgap in which the number of minority carriers is extremely small. Thus,thin film transistors can be manufactured using a large substrate, whichleads to the enhancement of mass production. Further, the use of theoxide semiconductor film in which the hydrogen concentration is reducedand the purity is increased makes it possible to manufacture a thin filmtransistor which is suitable for increase in definition and has highoperation speed and in which a large amount of current can flow when thethin film transistor is on and almost no current flows when the thinfilm transistor is off.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 6

In this embodiment, an element substrate and a pixel structure in adisplay device in which the field-effect transistor, for example thethin film transistor described in any of the above embodiments is usedwill be described with reference to FIG. 9 and FIG. 10.

FIG. 9 is a top view of a pixel 160 in a display portion in a displaydevice. FIG. 10 is a cross-sectional view taken along dashed line A-Band dashed line C-D in FIG. 9.

Although the thin film transistor 145 described in Embodiment 1 is usedas a pixel thin film transistor for controlling a potential of a pixelelectrode for description in this embodiment, any of the thin filmtransistors described in the other embodiments can be used asappropriate. The first electrode 105 that functions as one of a sourceelectrode and a drain electrode of the thin film transistor 145 isconnected to a pixel electrode 167. The second electrode 109 thatfunctions as the other of the source electrode and the drain electrodeof the thin film transistor 145 is connected to a signal line 161through a conductive film 165. In addition, a capacitor wiring 163 isformed of the layer that is used for forming the first electrode 105.The conductive film 165 and the pixel electrode 167 are formed over aplanarization insulating film 171 for planarization.

Note that in the thin film transistors described in Embodiments 1 to 6,the oxide semiconductor film in which the hydrogen concentration isreduced and the purity is increased is used; thus, off-state current ofthe thin film transistors is low. Thus, a capacitor for holding signalvoltage applied to the pixel electrode does not have to be additionallyprovided. In other words, the capacitor wiring 163 does not need to beprovided; thus, the aperture ratio of the pixel can be increased.

The planarization insulating film 171 can be formed using the materialof the planarization insulating film, which is described in Embodiment3, as appropriate.

The pixel electrode 167 is formed using a conductive film that isfavorable to each display device.

The element substrate described in this embodiment can be used asappropriate in other display devices such as a liquid crystal displaydevice, a light-emitting display device, and an electrophoretic displaydevice. In addition, the structure of the pixel is not limited to thestructure illustrated in FIG. 9 and FIG. 10, and a thin film transistor,a diode, and a capacitor can be provided as appropriate.

Since the thin film transistors described in Embodiments 1 to 5 can beminiaturized, a high-definition display device can be manufactured usingan element substrate described in this embodiment.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 7

In this embodiment, the case where a thin film transistor ismanufactured and the thin film transistor is used in a pixel portion anda peripheral circuit portion (e.g., a driver circuit) so that asemiconductor device having a display function (a display device) ismanufactured will be described. Part of or all the peripheral circuitportion is formed over a substrate where the pixel portion is formed,whereby a system-on-panel can be realized.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement), a light-emitting element (also referred to as a light-emittingdisplay element), or the like can be used. The light-emitting elementincludes, in its category, an element whose luminance is controlled bycurrent or voltage, and specifically includes, in its category, aninorganic electroluminescent (EL) element, an organic EL element, andthe like. Further, a display medium whose contrast is changed by anelectric effect, such as electronic ink, may be used.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC including a controller orthe like is mounted on the panel. Furthermore, an element substrateincluded in a display device is provided with a unit for supplyingcurrent to the display element in each of pixel portions. Specifically,the element substrate may be in a state after only a pixel electrode ofthe display element is formed, or in a state after a conductive layer tobe a pixel electrode is formed and before the conductive layer isetched.

Hereinafter, in this embodiment, an example of a liquid crystal displaydevice will be described. FIGS. 11A1 and 11A2 are plan views and FIG.11B is a cross-sectional view of a panel in which thin film transistors4010 and 4011 and a liquid crystal element 4013 that are formed over afirst substrate 4001 are sealed by a second substrate 4006 and a sealant4005. Here, FIGS. 11A1 and 11A2 are each a plan view and FIG. 11B is across-sectional view taken along line M-N in FIGS. 11A1 and 11A2.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 that are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scan line driver circuit 4004. In other words, thepixel portion 4002 and the scan line driver circuit 4004 are sealedtogether with a liquid crystal layer 4008, by the first substrate 4001,the sealant 4005, and the second substrate 4006. Further, a signal linedriver circuit 4003 that is formed using a single crystal semiconductoror a polycrystalline semiconductor over a substrate separately preparedis mounted in a region that is different from the region surrounded bythe sealant 4005 over the first substrate 4001.

Note that there is no particular limitation on the connection method ofa driver circuit that is separately formed, and a COG method, a wirebonding method, a TAB method, or the like can be used as appropriate.FIG. 11A-1 illustrates an example of mounting the signal line drivercircuit 4003 by a COG method, and FIG. 11A-2 illustrates an example ofmounting the signal line driver circuit 4003 by a TAB method.

Further, the pixel portion 4002 and the scan line driver circuit 4004provided over the first substrate 4001 each include a plurality of thinfilm transistors. FIG. 11B illustrates the thin film transistor 4010included in the pixel portion 4002 and the thin film transistor 4011included in the scan line driver circuit 4004. An insulating film 4020is provided over the thin film transistors 4010 and 4011.

As the thin film transistors 4010 and 4011, any of the thin filmtransistors that are described in the above embodiments, or the like canbe employed.

A pixel electrode 4030 included in the liquid crystal element 4013 iselectrically connected to the thin film transistor 4010. A counterelectrode 4031 of the liquid crystal element 4013 is provided for thesecond substrate 4006. The liquid crystal element 4013 is constituted bythe pixel electrode 4030, the counter electrode 4031, and the liquidcrystal layer 4008. Note that the pixel electrode 4030 and the counterelectrode 4031 are provided with an insulating film 4032 and aninsulating film 4033, respectively, each of which functions as analignment film. The liquid crystal layer 4008 is sandwiched between thepixel electrode 4030 and the counter electrode 4031 with the insulatingfilms 4032 and 4033 interposed therebetween.

Note that the substrate 101 that is described in Embodiment 1 can beused as the first substrate 4001 and the second substrate 4006, asappropriate. Alternatively, metal (typically stainless steel), ceramic,plastic, or the like can be used. As plastic, a fiberglass-reinforcedplastics (FRP) substrate, a polyvinyl fluoride (PVF) film, a polyesterfilm, an acrylic resin film, or the like can be used. Furtheralternatively, a sheet in which aluminum foil is sandwiched by PVF filmsor polyester films can be used.

A columnar spacer 4035 is provided in order to control the distance(cell gap) between the pixel electrode 4030 and the counter electrode4031. The columnar spacer 4035 can be obtained by selective etching ofan insulating film. Note that a spherical spacer may be used instead ofthe columnar spacer. The counter electrode 4031 is electricallyconnected to a common potential line formed over the same substrate asthe thin film transistor 4010. For example, the counter electrode 4031can be electrically connected to the common potential line throughconductive particles provided between the pair of substrates. Note thatthe conductive particles are preferably contained in the sealant 4005.

Alternatively, a liquid crystal showing a blue phase for which analignment film is unnecessary may be used. A blue phase is one of theliquid crystal phases, which is generated just before a cholestericphase changes into an isotropic phase while temperature of cholestericliquid crystal is increased. Since the blue phase is only generatedwithin a narrow range of temperatures, a liquid crystal compositioncontaining a chiral agent at 5 wt % or more is preferably used. Thus,the temperature range can be improved. The liquid crystal compositionwhich includes a liquid crystal showing a blue phase and a chiral agenthas a small response time of 101 μs to 100 μs, has optical isotropy,which makes the alignment process unneeded, and has a small viewingangle dependence.

Although an example of a transmissive liquid crystal display device isdescribed in this embodiment, the present invention is not limitedthereto, and a reflective liquid crystal display device or asemi-transmissive liquid crystal display device may be formed.

As the example of the liquid crystal display device described in thisembodiment, a polarizing plate is provided on the outer surface of thesubstrate (on the viewer side) and a coloring layer and an electrodeused for a display element are provided on the inner surface of thesubstrate; however, the polarizing plate may be provided on the innersurface of the substrate. The stacked structure of the polarizing plateand the coloring layer is not limited to this embodiment and may be setas appropriate depending on materials of the polarizing plate and thecoloring layer or conditions of manufacturing process. Further, a blackmask (a black matrix) may be provided as a light-shielding film.

Although the thin film transistor obtained in any of the aboveembodiments is covered with the insulating film 4020 in order to reducesurface unevenness caused by the thin film transistor in thisembodiment, the invention disclosed is not limited to this structure.

The insulating film 4020 can be formed using the material of theplanarization insulating film, which is described in Embodiment 3, asappropriate.

The pixel electrode 4030 and the counter electrode 4031 can be formedusing a light-transmitting conductive material such as indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, indium tin oxide (hereinafter referred to as ITO),indium zinc oxide, or indium tin oxide to which silicon oxide is added.

A conductive composition containing a conductive high molecule (alsoreferred to as a conductive polymer) may be used for the pixel electrode4030 and the counter electrode 4031. The pixel electrode formed of theconductive composition preferably has a sheet resistance of less than orequal to 1.0×10⁴Ω/square and a transmittance of greater than or equal to70% at a wavelength of 550 nm. Furthermore, the resistivity of theconductive high molecule contained in the conductive composition ispreferably less than or equal to 0.1 Ω·cm.

As the conductive high molecule, a so-called π-electron conjugatedconductive polymer can be used. For example, polyaniline or a derivativethereof, polypyrrole or a derivative thereof, polythiophene or aderivative thereof, a copolymer of two or more kinds of them, and thelike can be given.

A variety of signals are supplied from an FPC 4018 to the signal linedriver circuit 4003, the scan line driver circuit 4004, the pixelportion 4002, or the like.

In addition, a connection terminal electrode 4015 is formed from thesame conductive film as the pixel electrode 4030 included in the liquidcrystal element 4013, and a terminal electrode 4016 is formed from thesame conductive film as a source or drain electrode of the thin filmtransistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 through an anisotropic conductive film4019.

Note that FIGS. 11A1, 11A2 and 11B illustrate the example in which thesignal line driver circuit 4003 is formed separately and then mounted onthe first substrate 4001; however, this embodiment is not limited tothis structure. The scan line driver circuit may be separately formedand then mounted, or only part of the signal line driver circuit or partof the scan line driver circuit may be separately formed and thenmounted.

Since the thin film transistors described in Embodiments 1 to 5 can beminiaturized, a high-definition liquid crystal display device can bemanufactured.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 8

In this embodiment, active matrix electronic paper that is an example ofa semiconductor device will be described with reference to FIG. 12. Athin film transistor 650 used in a semiconductor device can bemanufactured in a manner similar to that of any of the thin filmtransistors described in the above embodiments.

The electronic paper illustrated in FIG. 12 is an example of a displaydevice in which a twist ball display method is employed. The twist balldisplay method refers to a method in which spherical particles eachcolored in black and white are arranged between a first electrode and asecond electrode, and a potential difference is generated between thefirst electrode and the second electrode, whereby orientation of twistballs is controlled so that display is performed.

The thin film transistor 650 provided over a substrate 600 is a thinfilm transistor according to one embodiment of the invention disclosedand has a structure in which an oxide semiconductor film is sandwichedbetween a source or drain electrode which is located above the oxidesemiconductor film and a source or drain electrode which is locatedbelow the oxide semiconductor film. Note that the source or drainelectrode is electrically connected to a first electrode 660 through acontact hole formed in an insulating film. A substrate 602 is providedwith a second electrode 670. Twist balls 680 each having a black region680 a and a white region 680 b are provided between the first electrode660 and the second electrode 670. A space around the twist balls 680 isfilled with a filler 682 such as a resin (see FIG. 12). In FIG. 12, thefirst electrode 660 corresponds to a pixel electrode, and the secondelectrode 670 corresponds to a common electrode. The second electrode670 is electrically connected to a common potential line provided overthe substrate where the thin film transistor 650 is formed.

Instead of the twist ball, an electrophoretic display element can beused. In that case, for example, a microcapsule having a diameter ofapproximately 10 μm to 200 μm in which transparent liquid,positively-charged white microparticles, and negatively-charged blackmicroparticles are encapsulated, is used. When an electric field isapplied by the first electrode and the second electrode, the whitemicroparticles and the black microparticles move to opposite sides fromeach other, whereby a white or black image is displayed. Theelectrophoretic display element has higher reflectance than a liquidcrystal display element; thus, an auxiliary light is unnecessary and adisplay portion can be recognized in a place where brightness is notsufficient. In addition, there is an advantage that an image that hasbeen displayed once can be maintained even when power is not supplied tothe display portion.

As described above, the use of the invention disclosed makes it possibleto manufacture high-performance electronic paper. This embodiment can beimplemented in appropriate combination with the structures described inthe other embodiments.

Since the thin film transistors described in Embodiments 1 to 5 can beminiaturized, high-definition electronic paper can be manufactured.

Embodiment 9

In this embodiment, an example of a light-emitting display device willbe described as a semiconductor device. As a display element included ina display device, a light-emitting element utilizing electroluminescencewill be described here. Light-emitting elements utilizingelectroluminescence are classified according to whether a light-emittingmaterial is an organic compound or an inorganic compound. In general,the former is referred to as an organic EL element, and the latter isreferred to as an inorganic EL element.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. Then, the carriers (electrons and holes) recombine,so that light is emitted. Owing to this mechanism, the light-emittingelement is called a current-excitation light-emitting element.

The inorganic EL elements are classified, according to their elementstructures, into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that an example inwhich an organic EL element is used as a light-emitting element isdescribed here.

Next, the appearance and a cross section of a light-emitting displaypanel (also referred to as a light-emitting panel), which corresponds toone embodiment of the semiconductor device, are described with referenceto FIGS. 13A and 13B. FIG. 13A is a plan view and FIG. 13B is across-sectional view of a panel in which thin film transistors 4509 and4510 and a light-emitting element 4511 that are formed over a firstsubstrate 4501 are sealed by a second substrate 4506 and a sealant 4505.Here, FIG. 13A is a plan view and FIG. 13B is a cross-sectional viewtaken along line H-I in FIG. 13A.

The sealant 4505 is provided to surround a pixel portion 4502, signalline driver circuits 4503 a and 4503 b, and scan line driver circuits4504 a and 4504 b, which are provided over the first substrate 4501. Inaddition, the second substrate 4506 is provided over the pixel portion4502, the signal line driver circuits 4503 a and 4503 b, and the scanline driver circuits 4504 a and 4504 b. In other words, the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b are sealed together with afiller 4507, with the first substrate 4501, the sealant 4505, and thesecond substrate 4506. Packaging (sealing) is preferably performed, insuch a manner, using a protective film (e.g., a bonding film or anultraviolet curable resin film), a cover material, or the like with highair-tightness and little degasification.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b, which are formedover the first substrate 4501, each include a plurality of thin filmtransistors. FIG. 13B illustrates the thin film transistor 4510 includedin the pixel portion 4502 and the thin film transistor 4509 included inthe signal line driver circuit 4503 a.

As the thin film transistors 4509 and 4510, any of the thin filmtransistors described in the above embodiments can be employed.

A first electrode 4517 that is a pixel electrode of the light-emittingelement 4511 is electrically connected to a source electrode or a drainelectrode of the thin film transistor 4510. Note that the structure ofthe light-emitting element 4511 is not limited to the stacked structurein this embodiment, which includes the first electrode 4517, alight-emitting layer 4513, and a second electrode 4514. The structure ofthe light-emitting element 4511 can be changed as appropriate dependingon the direction in which light is extracted from the light-emittingelement 4511, or the like.

As for the first electrode 4517 and the second electrode 4514, anelectrode that functions as a cathode can be formed using a conductivefilm that has a small work function and reflects light. For example, theelectrode that functions as a cathode is preferably formed using amaterial such as Ca, Al, MgAg, or AlLi. An electrode that functions asan anode is formed using a light-transmitting conductive material. Forexample, a light-transmitting conductive material such as indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, indium tin oxide, indium zinc oxide, or indium tin oxideto which silicon oxide is added may be used.

A partition 4520 is formed using an organic resin film, an inorganicinsulating film, organic polysiloxane, or the like. It is particularlypreferable that the partition 4520 be formed using a photosensitivematerial to have an opening over the first electrode 4517 so that asidewall of the opening is formed as an inclined surface with continuouscurvature.

The light-emitting layer 4513 may be formed using a single layer or aplurality of layers stacked.

A protective film may be formed over the second electrode 4514 and thepartition 4520 in order to prevent oxygen, hydrogen, water, carbondioxide, or the like from entering the light-emitting element 4511. Asthe protective film, a silicon nitride film, a silicon nitride oxidefilm, a DLC film, or the like can be formed.

A variety of signals are supplied from FPCs 4518 a and 4518 b to thesignal line driver circuits 4503 a and 4503 b, the scan line drivercircuits 4504 a and 4504 b, the pixel portion 4502, or the like.

In this embodiment, an example is described in which a connectionterminal electrode 4515 is formed using the same conductive film as thefirst electrode 4517 of the light-emitting element 4511, and a terminalelectrode 4516 is formed using the same conductive film as the source ordrain electrode of the thin film transistors 4509 and 4510.

The connection terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a via an anisotropic conductive film4519.

The substrate located in the direction in which light is extracted fromthe light-emitting element 4511 needs to have a light-transmittingproperty. As a substrate having a light-transmitting property, a glassplate, a plastic plate, a polyester film, an acrylic film, and the likeare given.

As the filler 4507, an ultraviolet curable resin, a thermosetting resin,or the like can be used, in addition to an inert gas such as nitrogen orargon. For example, polyvinyl chloride (PVC), acrylic, polyimide, anepoxy resin, a silicone resin, polyvinyl butyral (PVB), ethylene vinylacetate (EVA), or the like can be used. In this embodiment, an examplein which nitrogen is used for the filler is described.

If needed, an optical film, such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter, may be provided on a light-emitting surface of thelight-emitting element. Furthermore, an antireflection treatment may beperformed on a surface thereof. For example, anti-glare treatment bywhich reflected light can be diffused by projections and depressions onthe surface so as to reduce the glare can be performed.

The signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b may be formed using a single crystalsemiconductor or a polycrystalline semiconductor over a substrateseparately prepared. Alternatively, only the signal line driver circuitsor part thereof or only the scan line driver circuits or part thereofmay be separately formed and mounted. This embodiment is not limited tothe structure illustrated in FIGS. 13A and 13B.

Through the above-described steps, a high-definition light-emittingdisplay device (display panel) can be manufactured. This embodiment canbe implemented in appropriate combination with the structures describedin the other embodiments.

Embodiment 10

In this embodiment, examples of electronic devices each including thedisplay device described in the above embodiment will be described.

FIG. 14A illustrates a portable game machine, which includes a housing9630, a display portion 9631, a speaker 9633, operation keys 9635, aconnection terminal 9636, a memory medium insert portion 9672, and thelike. The portable game machine illustrated in FIG. 14A has a functionof reading a program or data stored in the recording medium to displayit on the display portion, a function of sharing information withanother portable game machine by wireless communication, and the like.Note that the portable game machine illustrated in FIG. 14A can have avariety of functions without limitation to the above-describedfunctions.

FIG. 14B illustrates a digital camera, which includes the housing 9630,the display portion 9631, the speaker 9633, the operation keys 9635, theconnection terminal 9636, a shutter button 9676, an image receivingportion 9677, and the like. The digital camera with a televisionreception function illustrated in FIG. 14B has a function ofphotographing a still image and/or a moving image, a function ofautomatically or manually correcting the photographed image, a functionof obtaining various kinds of information from an antenna, a function ofstoring the photographed image or the information obtained from theantenna, a function of displaying the photographed image or theinformation obtained from the antenna on the display portion, and thelike. Note that the digital camera with the television receptionfunction illustrated in FIG. 14B can have a variety of functions withoutlimitation to the above-described functions.

FIG. 14C illustrates a television set, which includes the housing 9630,the display portion 9631, the speakers 9633, the operation keys 9635,the connection terminal 9636, and the like. The television setillustrated in FIG. 14C has a function of converting an electric wavefor television into an image signal, a function of converting the imagesignal into a signal suitable for display, a function of converting aframe frequency of the image signal, and the like. Note that thetelevision set illustrated in FIG. 14C can have a variety of functionswithout limitation to the above-described functions.

FIG. 15A illustrates a computer, which includes the housing 9630, thedisplay portion 9631, the speaker 9633, the operation keys 9635, theconnection terminal 9636, a pointing device 9681, an external connectionport 9680, and the like. The computer illustrated in FIG. 15A has afunction of displaying a various kinds of information (e.g., a stillimage, a moving image, and a text image) on the display portion, afunction of controlling processing by various kinds of software(programs), a communication function such as wireless communication orwired communication, a function of being connected to a variety ofcomputer networks with the communication function, a function oftransmitting or receiving a variety of data with the communicationfunction, and the like. Note that the computer illustrated in FIG. 15Acan have a variety of functions without limitation to theabove-described functions.

FIG. 15B illustrates a mobile phone, which includes the housing 9630,the display portion 9631, the speaker 9633, the operation keys 9635, amicrophone 9638, and the like. The mobile phone illustrated in FIG. 15Bhas a function of displaying various kinds of information (e.g., a stillimage, a moving image, and a text image) on the display portion, afunction of displaying a calendar, a date, the time, or the like on thedisplay portion, a function of operating or editing the informationdisplayed on the display portion, a function of controlling processingby various kinds of software (programs), and the like. Note that themobile phone illustrated in FIG. 15B can have a variety of functionswithout limitation to the above-described functions.

FIG. 15C illustrates a device including electronic paper (the device isalso referred to as an e-book reader), which includes the housing 9630,the display portion 9631, the operation keys 9635, and the like. Thee-book reader illustrated in FIG. 15C has a function of displayingvarious kinds of information (e.g., a still image, a moving image, and atext image) on the display portion, a function of displaying a calendar,a date, the time, or the like on the display portion, a function ofoperating or editing the information displayed on the display portion, afunction of controlling processing by various kinds of software(programs), and the like. Note that the electronic paper illustrated inFIG. 15C can have a variety of functions without limitation to theabove-described functions.

In each of the electronic devices described in this embodiment,off-state current can be reduced in a plurality of pixels included inthe display portion. Thus, an electronic device that includes a displaydevice which consumes less power can be obtained. Further, when apertureratio is increased, the display device can include a high-definitiondisplay portion.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

This application is based on Japanese Patent Application serial no.2009-251065 filed with Japan Patent Office on Oct. 30, 2009, the entirecontents of which are hereby incorporated by reference.

1. A transistor comprising: a first electrode over a substrate; an oxidesemiconductor film on and in contact with the first electrode; a secondelectrode on and in contact with the oxide semiconductor film; a gateinsulating film on at least side surfaces of the oxide semiconductorfilm; and a third electrode having a ring shape, the third electrodeadjacent to the side surfaces of the oxide semiconductor film with thegate insulating film interposed therebetween.
 2. The transistoraccording to claim 1, wherein the first electrode functions as one of asource electrode and a drain electrode, wherein the second electrodefunctions as the other of the source electrode and the drain electrode,and wherein the third electrode functions as a gate electrode.
 3. Thetransistor according to claim 1, wherein carrier concentration of theoxide semiconductor film is lower than or equal to 5×10¹⁴/cm³.
 4. Thetransistor according to claim 1, wherein hydrogen concentration of theoxide semiconductor film is lower than or equal to 5×10¹⁹/cm³.
 5. Adisplay device comprising the transistor according to claim 1, whereinthe display device is incorporated into one selected from the groupconsisting of a portable game machine, a digital camera, a televisionset, a computer, a mobile phone and a device including electronic paper.6. A transistor comprising: a first electrode over a substrate; an oxidesemiconductor film on and in contact with the first electrode; a secondelectrode on and in contact with the oxide semiconductor film; a gateinsulating film on at least side surfaces of the oxide semiconductorfilm; a third electrode having a ring shape, the third electrodeadjacent to the side surfaces of the oxide semiconductor film with thegate insulating film interposed therebetween; and an interlayerinsulating film over the third electrode.
 7. The transistor according toclaim 6, wherein the first electrode functions as one of a sourceelectrode and a drain electrode, wherein the second electrode functionsas the other of the source electrode and the drain electrode, andwherein the third electrode functions as a gate electrode.
 8. Thetransistor according to claim 6, wherein carrier concentration of theoxide semiconductor film is lower than or equal to 5×10¹⁴/cm³.
 9. Thetransistor according to claim 6, wherein hydrogen concentration of theoxide semiconductor film is lower than or equal to 5×10¹⁹/cm³.
 10. Thetransistor according to claim 6, wherein the interlayer insulating filmcomprises a material selected from the group consisting of a siliconoxide film, a silicon oxynitride film, an aluminum oxide film, analuminum oxynitride film, a silicon nitride film, a silicon nitrideoxide film, an aluminum nitride film and an aluminum nitride oxide film.11. An display device comprising the transistor according to claim 6,wherein the display device is incorporated into one selected from thegroup consisting of a portable game machine, a digital camera, atelevision set, a computer, a mobile phone and a device includingelectronic paper.
 12. A transistor comprising: a first electrode over asubstrate; an island-shaped oxide semiconductor film on and in contactwith the first electrode; a second electrode on and in contact with theisland-shaped oxide semiconductor film; a gate insulating film on atleast side surfaces of the island-shaped oxide semiconductor film; and athird electrode covering the side surfaces of the island-shaped oxidesemiconductor film with the gate insulating film interposedtherebetween.
 13. The transistor according to claim 12, wherein thefirst electrode functions as one of a source electrode and a drainelectrode, wherein the second electrode functions as the other of thesource electrode and the drain electrode, and wherein the thirdelectrode functions as a gate electrode.
 14. The transistor according toclaim 12, wherein carrier concentration of the oxide semiconductor filmis lower than or equal to 5×10¹⁴/cm³.
 15. The transistor according toclaim 12, wherein hydrogen concentration of the oxide semiconductor filmis lower than or equal to 5×10¹⁹/cm³.
 16. A display device comprisingthe transistor according to claim 12, wherein the display device isincorporated into one selected from the group consisting of a portablegame machine, a digital camera, a television set, a computer, a mobilephone and a device including electronic paper.